HoneyComb's right - you don't get much out of volunteering for jobs, and it's generally better to fly under the radar as much as possible. Myself, I landed a job as "Port Watch" - responsible (in an integrated division) for writing the men's watchbill. Naturally, I didn't stand any watch on "liberty weekend" after we graduated, but the nightly headaches from putting up with whiners made it hardly worth it. No, far better to just stay low. Don't worry about PT, either...it's all about will-power; if you have any at all, you'll be fine.
The only thing I'd recommend volunteering for in all of boot camp is for a 900 division (nicknamed "band camp"). If you enjoy playing musical instruments (as I do), you'll find practices are a pleasant break from what's otherwise a pretty boring couple months, and your pupils usually already know how to march, which results in fewer drills and YFG speeches. Oh, and go to church, even if you aren't religious. If nothing else, you can sleep.